1. Field of the Invention
This invention relates to a bulk composition suitable for counting individual neutrons. More particularly, the present invention is drawn to a specific material which is sensitive to neutron radiation and which can comprise a semiconducting substrate to act as a 3-dimensional detector array.
2. Description of the Prior Art
Conventional neutron detectors typically include devices that operate as ionization chambers or proportional counters, both of which use a neutron active gas such as BF3 or He. Upon absorption of neutrons, such gases release energetic reaction particles. These particles produce ionization in the surrounding gas which are detected by appropriately biased electrodes. Other detectors coat the walls of the ionization chamber with a solid neutron active material such as 6Li, 10B, or 235U. These materials also absorb neutrons and release particles that produce ionization.
One class of solid state neutron detectors detect electron-hole pairs that cross a semiconductor junction. The electron-hole pairs are produced by reaction particles formed as result of neutron absorption within films or dopants of neutron active material incorporated within the detector. One such solid state neutron detector is disclosed in U.S. Pat. No. 3,227,876 to Ross, which includes a silicon semiconductor having a layer doped with boron. Neutrons are absorbed by the boron layer, thereby creating energetic reaction particles that, in turn, create electron-hole pairs that diffuse into and across the junction to produce a current pulse. The detector may be encapsulated by a layer of hydrogenous moderator material a few centimeters thick in order to reduce the speed of incoming neutrons. Such detectors are susceptible to radiation damage and are not capable of operating at temperatures above 30xc2x0 or 40xc2x0 C. for extended periods of time, making them unsuitable for use in high temperature, high radiation environments.
U.S. Pat. No. 3,805,078 to Kozlov discloses a solid state neutron detector including at least one layer of diamond crystal.
U.S. Pat. No. 4,419,578 to Kress discloses another solid state neutron detector that uses a hydrogen containing semiconductor material.
A major problem with prior art neutron detectors is the sensitivity of the detector to non-neutronic components of the radiation field, particularly gamma ray sensitivity. Lithium glass scintillators, although generally less efficient, are an effective means for detecting low-energy neutrons and find wide application in neutron scattering research. However, lithium glass scintillators also suffer from a sensitivity to gamma-rays where the presence of a background radiation is large in relationship to a flux of neutrons. In such instances, the gamma sensitivity of Li-glass simulates a neutron capture event in Li-glass and since there is no effective technique for separating the gamma signal from the neutron signal (for coincidental multiple photon events with total energy deposition in the vicinity of the capture peak) the quality of the data obtained is seriously degraded.
Neutron scattering research facilities require a detector system that is efficient, fast, and gamma insensitive. None of the detector systems currently used by researchers meet all these requirements.
In accordance with the principles of the invention, a new neutron detector has been developed which overcomes the disadvantages of the prior art scintillation or gas-phase detectors. The neutron detector in accordance with the invention relies upon single or polycrystalline, boron-containing compounds, useful for neutron detection. The 10B(n, alpha) reaction possesses a large cross section for neutron capture and produces nuclear decay fragments which are at once heavy and energetic. The present invention takes advantage of the relatively short distances over which the energy of these heavy ion decay products is dissipated within crystals of the boron-containing compound to permit using moderately thin though still structurally robust wafers for detecting the presence of neutrons. Also possible are articles prepared from powders of the boron compound prepared by the instant process: thin sintered wafers prepared by comminuting the boron compound of the present invention to a powder, mixing the powder with binder agents and sintering it, or thin layers of pastes prepared by mixing a comminuted powder of the boron compound with wetting and/or dispersing agents and laid down by a printing process.
Because of the reduced need for large volume crystals the detector is rendered inherently less sensitive to background gamma-ray radiation due to the comparatively large mean-free-path lengths for these energetic photons to dissipate their energy. Thin or very small crystals result in most of this radiation passing undetected through these crystals and thereby strongly discriminating against gamma background events.
The borate materials described in the instant invention provide a high boron atomic density, and is conveniently formed into single or polycrystalline boules. The specific variation of these materials which is useful for providing the desired neutron detection response is prepared by INRAD Corp. in a process which avoids the use of a flux, typically sodium oxide or a fluoride, in the growth of the crystal boules. Crystalline material fabricated by standard industry processes using sodium oxide or fluoride fluxes have been found to provide virtually no response to impressed neutron radiation. It is believed, therefore, that the observed behavior of crystal boules fabricated by the fluxless process is the result of substantially reduced impurity levels.